According to Wikipedia, “lens flare” is, “the light scattered in lens systems through generally unwanted image formation mechanisms, such as internal reflection and scattering from material inhomogeneities in the lens.” This definition implies that it’s unwanted all the time, and that’s not true in photography. Flare can be used to add drama, to introduce a balancing compositional element, and to help tell a story.
In this image, of Stonehenge in Wiltshire, UK, I have shot directly towards the sun. The lens flare introduces colour (in the form of blue circles), produces sun rays on the right-hand side, and gives a contrast change across the frame. It helps with the narrative of the image, as Stonehenge is believed to be a sort of solar observatory/calendar. It needs the sun to fulfill its purpose.
Likewise this image, at a kite festival, uses flare to add to the composition. Without the sun heating up the earth’s atmosphere there would be no wind, so a kite needs the sun. Obviously the sun’s disc is completely overexposed, but there are attractive diffraction rays around it, and colour patterns produced by the massive amount of light being bounced around inside the lens. There’s a darker highlight in the top of the frame which helps to produce a nice diagonal line from the bottom right to the top left.
So flare can add flair to your images.
Caveat: As with any activity that involves looking towards the sun, please remember that direct sunlight can damage your eyes. Only look at the sun indirectly – such as on your camera’s rear screen. I used a compact camera for these images, but if you use a DSLR that doesn’t have live view be extra careful.